When electrically connecting between electronic components with a bonding material composed of a metal such as a solder, the bonding material is first heat-melted to bond the electronic components to each other, and then cooled until solidification occurs. As is known, the bonding material expands during the heat-melting, and contracts during the cooling. Where the bonding material has a high strength, however, an internal stress induced in a connecting portion formed of the bonding material cannot resiliently be absorbed by the connecting portion alone. In addition, the connecting portion suffers from stresses such as thermal fatigue due to heat from an ambient temperature and other environmental conditions as well as heat generated by the components during operation. This may result in cracking of the connecting portion, breakage or separation of the interconnection between the electronic components, or breakage of an insulating layer, which in turn leads to breakdown of the electronic components interconnected by the bonding material. Therefore, the reliability of the whole electronic system may be significantly reduced.
There have been proposed a variety of approaches to the aforesaid problem, one of which is to use a soft metal as the bonding material. Conventionally, lead is mainly used for this purpose. Although lead is excellent in physical properties, limitations are imposed due to environmental problems. Indium is also a typical soft metal which is comparable to lead. However, indium is expensive because of its low production, and has a lower melting point, so that applications thereof are limited.
Another approach is to use a metal-coated resin ball as the bonding material. For example, a solder-plated resin ball is used as a bonding material for a BGA (ball grid array) module. However, the solder-plated resin ball has a relatively large continuous resin-solder interface, so that cracking due to fatigue may propagate along the interface.
It is believed that a bonding material and resulting interconnection structure that overcomes the aforementioned problems will represent significant contributions to the art.